Method and apparatus for preventing bacteria proliferation in an electric water heater

ABSTRACT

A method and an apparatus is described for preventing bacteria proliferation in a bottom end of a water holding tank of an electric water heater. Different water convection devices are described for convecting heated water from inside the tank and into the bottom end of the tank to raise the temperature at the bottom end sufficiently high to prevent bacteria growth and particularly at a temperature of at least 46 degrees C. where the  Legionella  bacteria can not survive. The temperature at the bottom end of the tank is monitored and the convection devices controlled to maintain the desired hot water temperature. An added benefit of the invention is that it also prevents sediment deposits and water temperature stratification in the tank bottom end section.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for preventingbacteria proliferation, and particularly but not exclusively, theLegionella bacteria, in an electric water heater by the use of heatedwater within the tank of the water heater at a temperature sufficientlyhigh to kill the bacteria.

BACKGROUND OF THE INVENTION

Although some species of Legionella bacteria can be found in the soil,most species live in water that is stagnant and wherein such bacteriasurvive under a wide range of temperatures, typically 20 to 45 degreesC. According to the Centers for Disease Control and Prevention, USA,between 8,000 and 18,000 people are hospitalized with Legionnairesdisease each year. It is of great public concern as its fatality rateduring an outbreak ranges from 5% to 30% in those who contract thedisease. Actively managing the risk of Legionella in water systems ismore cost effective than responding to an outbreak. Outbreaks ofLegionella pneumophila can stem from showers and potable water systems.As water from such sources aerosolized, individuals can inhale theLegionella containing droplets and the organism is aspirated into thelungs.

The formation and multiplication of such Legionella bacteria is not onlypromoted by the temperature in the customary hot water systems, but alsoby the fact that dead spaces are present in such water distributionsystems in which deposits and sediment formation can arise, andtypically in the bottom zone of water heater tanks. Deposits therein canrepresent a culture medium for bacteria proliferation.

Most electric water heaters for domestic use have its water tankconstructed with a dome shaped bottom wall. Such dome-shaped bottomwalls form a surrounding cavitated zone about the dome-shaped wall wheresediments deposit can gather and where water is less agitated. Thiscavitated zone is spaced from the bottom heating element and thus watertherein is less hot creating an ideal location for bacterialproliferation. Should the bottom element fail, then the watertemperature at the bottom of the tank will drop. It as also beendetermined by research that the Legionella bacteria does not survive attemperatures above 46 degrees C. When hot water is not drawn from awater heater, the water inside the tank becomes stagnant and the watertemperature stratifies with the cooler temperature being at the bottomregion of the tank. Water within the cavitated zone below the bottomelement of the tank can fall to about 30 to 40 degrees C. which isfavourable to bacteria growth. Lowering the bottom element to place itclose to the bottom wall of the tank has not proven to be a viablesolution.

Reference is made to U.S. Pat. Nos. 4,940,024; 5,168,546 and 5,808,277which disclose various methods and apparatus to prevent bacteriaproliferation in electric water heaters. One method teaches adding aheating element in the form of a belt or patch on the outside of thetank against the bottom end of the outer sidewall of the tank to heatthe water at the bottom end of the tank to a temperature preferablyabove 55 degrees C. Accordingly, this proposed solution provides anextra heating element in the form of a patch heater located in an areawhich is usually filled with insulating foam material and not practicalto access should it fail and require replacement or repair. It is alsocostly and consumes more electricity. In U.S. Pat. No. 5,808,277 a thirdelement is added into the tank to periodically raise the watertemperature at the bottom of the tank beyond the pre-set consumptiontemperature, to a sanitizing temperature to destroy bacteria. This isalso a costly proposition. U.S. Pat. No. 4,940,024 discloses a method ofdirecting the cold water flow of all consumed drinking or domesticallyused water through the lower region of the tank wherein there is nostagnant water and wherein no deposits can be formed for bacteriagrowth. Accordingly, the lower region of the tank is continuouslyflushed with fresh water. This is a costly solution requiring a new tankdesign and cold water conduit network and therefore not a viablesolution.

SUMMARY OF THE INVENTION

It is a feature of the present invention to provide a method andapparatus for the prevention of the Legionella bacteria in an electricwater heater which is different from the known prior art and which usesheated water within the water tank to elevate the temperature of thewater in the lower region of the tank sufficiently high to kill andprevent proliferation of bacteria.

Another feature of the present invention is to provide a pump, a watertemperature sensor and a control to pump hot water from the upper regionof the tank to the bottom end to maintain the temperature in the bottomend of the tank of an electric water heater at a temperature sufficientto kill and prevent proliferation of the Legionella bacteria whilesimultaneously preventing temperature stratification in the lower regionof the tank.

A further feature of the present invention is to provide a baffle platecombined with the bottom electrical resistive heating element of anelectric water heater to prevent the proliferation of the Legionellabacteria in the bottom end of the tank.

A still further feature of the present invention is to pump hot waterfrom the upper region of the tank of an electrical water heater and torelease it in the immediate area of the bottom wall of the tank to raisethe temperature of the water above 46 degrees C. whereby to killbacteria.

According to the above features, from a broad aspect, the presentinvention provides an electric water heater comprising a tank defined bya cylindrical side wall, a top wall and a dome-shaped bottom wall. Acold water inlet is disposed for releasing cold water under pressure ina lower portion of the tank. Two or more resistive heating elements heatwater in a respective one of an upper and lower region of the tank.Temperature sensing means is provided for sensing water temperature inthe upper and lower regions of said tank. Control means actuates atleast one of the two or more resistive heating elements when the watertemperature in at least one of the tank regions falls below a set pointtemperature value as sensed by the sensing means. The dome-shaped bottomwall has a surrounding cavitated zone thereabout where water temperatureis at its lowest and water is less agitated and more prone to bacteriaproliferation. Convection means is provided to cause heated water fromthe tank to be released in the lower region of the tank in the immediatearea of the dome-shaped bottom wall and he surrounding bacteriaproliferation zone at a temperature sufficient to kill and preventbacteria proliferation.

According to the broad aspect mentioned above, the convection means isconstituted by a water pump having an inlet conduit extending into theupper region of the tank where water is at a high temperature. The waterpump has an outlet conduit connected thereto which has a discharge enddisposed to release hot water from the upper region of the tank into thelower region in the immediate area of the dome-shaped bottom wall andinto a cavitated zone to raise the water temperature therein.

According to the broad aspect mentioned above, the convection means isconstituted by an inverted shallow cone-shaped baffle plate having anouter circular peripheral edge. The baffle plate is retained in thelower region of the tank and houses a substantial portion of a lower oneof the resistive heating elements extending in the tank lower region.The resistive heating element is disposed in a chamber integrally formedalong a top end section of the cone-shaped baffle plate and incommunication with the space under the baffle plate immediately abovethe dome-shaped bottom wall. The chamber has an elongated slot openingfor releasing hot water from under the baffle plate heated by theresistive heating element and causing hot water to flow under the baffleplate against the bottom wall.

According to a still further broad aspect of the present invention,there is provided a method of preventing bacterial growth in a bottomend of a water holding tank of an electric water heater in the region ofa dome-shaped bottom wall defining a cavitated zone thereabout. Themethod comprises the step of convecting heated water from above thedome-shaped bottom wall to the immediate area of the dome-shaped bottomwall and into the cavitated zone to maintain the temperature of water inthe cavitated zone at a temperature sufficient to prevent harmfulbacteria growth.

According to a further broad aspect of the invention, the method furthercomprises before the step of convection, sensing water temperature inthe immediate area of the dome-shaped bottom wall, and after the step ofconvection, controlling the duration of the step of convection inrelation to the sensed temperature to maintain water temperaturesufficiently high to prevent bacteria growth.

DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be describedwith reference to the examples of the preferred embodiment wherein:

FIG. 1 is a simplified section view of an electric water heater showingsome of the components thereof and illustrating one example of thepreferred embodiment wherein the convection means is a water pump whichdisplaces hot water from the top region of the tank to the lower regionthereof closely spaced to the dome-shaped bottom wall of the tank;

FIG. 2 is a fragmented side view illustrating a modification to theoutlet conduit of the pump and its relation to the dome-shaped bottomwall;

FIG. 3 is a top view illustrating the modification to the bottom endsection of the outlet conduit of FIG. 2;

FIG. 4 is a fragmented sectional side view illustrating a furtherexample of the convection means which is herein constituted by aninverted dome shaped baffle plate housing a lowermost one of theresistive heating elements to create a hot water flow in the immediatearea of the dome-shaped bottom wall of the tank;

FIG. 5 is a fragmented sectional side view, similar to FIG. 4, butproviding a transverse view of the inverted dome shaped baffle plate asshown in FIG. 4 and the lowermost resistive heating element;

FIG. 6 is a top perspective view of an example of the construction ofthe inverted dome shaped baffle plate, and

FIG. 7 is a fragmented section view showing the position of the outerperipheral edge of the inverted dome shape baffle plate and its relationto the side wall of the tank and the cavitated peripheral zone of thedome-shaped bottom wall of the tank and further illustrating one of thespacer clips secured to the peripheral edge of the baffle plate tocenter the inverted dome shape baffle plate with respect to the circularside wall of the tank.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring to the drawings and more particularly to FIGS. 1 to 3, thereis shown generally at 10 an electric water heater which is comprised ofa water holding steel tank 11 formed by a surrounding side wall 12, atop wall 13 and a dome-shaped bottom wall 14. A resistive heatingelement 15 is mounted to the tank side wall and projects in an upperregion 16 of the tank 12. A bottom resistive heating element 17 is alsomounted to the tank wall 14 and projects into a lower region 18 of thetank spaced above the bottom wall 14. A control thermistor 19 isequipped with a temperature sensor 20 and operates the resistive heatingelement 15 to maintain a set temperature value in the upper region 16 ofthe tank. Similarly, a control thermistor 21 and temperature sensor 22control the lower resistive heating element 17 and set at the sametemperature as the upper element. Typically, during ordinary operatingconditions the consumption temperature of the water in the tank 11 is at60 degrees C.

As previously described, the Legionella bacteria can survive in stagnantwater at temperatures of 20 to 45 degrees C. but above thesetemperatures the bacteria is destroyed. Because the tank bottom wall 14is formed as a dome, it defines a surrounding cavitated zone 23 aboutthe dome adjacent the tank side wall inner surface 24 wherein sedimentsin the water can build up as the water therein is less agitated by themovement of water in the tank as the water is heated and rises in thetank and also by the draw of cold water into the tank through the diptube 25 bottom end, space well above the bottom wall, as hot water isremoved from the upper region 16 through the outlet pipe 26. In order toprevent the proliferation of the Legionella bacteria in the bottom end30 of the tank below the lower resistive heating element 17 andparticularly in the cavitated zone 23, there is provided a small waterpump 27 conveniently mounted on the top wall 28 of the outer shroud 29of the tank 11 to pump water from the upper region 16 of the tank to thebottom end 30 in close proximity to the bottom wall. The water releasedin the bottom end is typically in the range of between 50 degrees C. to60 degrees C.

As shown in FIG. 1, the pump 27 has an inlet conduit 31 which extendsinto the upper region 16 of the tank to draw hot water therefrom whenthe pump is actuated by a control device 32 which is connected to atemperature sensor 33 secured against the tank side wall 11 adjacent thebottom end 30 close to the cavitated zone 23. The pump 27 also has anoutlet conduit 34 in the form of a dip tube which extends within thetank 11 and has its discharge end 35 disposed in the bottom end of thetank to release hot water from the upper region to the immediate area ofthe bottom wall and into the cavitated zone 23 to raise the watertemperature therein above 46 degrees C.

It is pointed out that the pump pressure is sufficiently low not toprecipitate sediment deposits from the cavitated zone into the upperportion of the tank to prevent mixture thereof with hot water drawn fromthe tank as such could cause malfunction of the mixing valves ofappliances connected to the hot water distribution conduits network. Bypositioning the outlet conduit 34 within the tank in contact with hotwater therein there is substantially no heat loss of the transfer of hotwater from the upper region 16 to the bottom end 30. The outlet conduit34 is also constructed from non-oxidation material, similar to the diptube 25 and capable of withstanding temperatures well above 60 degreesC. The conduits 31 and 34 are removably secured to the top wall 13 ofthe tank through suitable leak-proof bushings 31′ and 34′, respectively.

As shown in FIGS. 2 and 3, the outlet conduit can also be formed with acurved conduit end section 36 and oriented to cause a gentle swirlingmotion of the water over the dome-shaped bottom wall 14.

With reference now to FIGS. 4 to 7, there is illustrated another exampleof the preferred embodiment wherein the convection means is constitutedby an inverted shallow cone shape baffle plate 40, as shown inperspective view in FIG. 6. The baffle plate 40 has an outer peripheraledge 41 and is of circular contour whereby to be retained in the lowerregion or bottom end 30 of the tank and closely spaced to the innersurface 24 of the tank sidewall 12. A resistive heating element chamber42 is formed in the upper region of the baffle plate for housing thelower resistive heating element 17. This chamber 42 is formed as anopen-ended channel 42′ which is in communication with the space 43 underthe baffle plate 40 and above the dome-shaped bottom wall 14.

The open-ended channel 42′ has opposed parallel side walls 44 each ofwhich has an inwardly inclined top section 45 to define therebetween andelongated slot opening 46. The channel 42′ has a rear open end 47 and aclosed upwardly slopped front end 48. The lower resistive heatingelement 17 is received in the channel 42′ through the open end 47 andspaced from the side walls 44. The arms 50 of the resistive heatingelement 17 may be disposed in a vertical plane, as illustrated, or atany angle in the channel 42′, depending on the type of resistive heatingelement.

As illustrated, the diameter of the baffle plate 40 is slightly smallerthan the inner diameter of the tank side wall 12 whereby to define aspace or a circumferential passage 57 between the outer peripheral edge41 of the baffle plate 40 and the inner surface 24 of the tank side wall12. In order to ensure that the baffle plate 40 is centered with theinner diameter of the tank inner side wall 24, spacer members 58, asshown in FIGS. 4 and 5, are secured spaced apart about an outerperipheral section of the inverted cone shape baffle plate 40 andproject beyond the outer circular peripheral edge 41 for frictionalengagement with the inner surface 24 of the tank side wall 12. Becausethe inner surface 24 of the tank side wall 12 is coated with a glasslining 59, the spacer members 58 are formed with soft spring arms 60having a curved shape for smooth resilient contact with the glass liningto prevent damage to the glass coating. They also center the baffleplate 40 with respect to the inner diameter of the tank. Of course, thebaffle plate is installed and centered before the dome-shaped bottomwall is welded to the tank cylindrical side wall 12 and positioned toalign the open end 47 of the channel 42’ with the resistive heatingelement cavity. The resistive heating element 17 may be installed in thechannel 42′ before installation of the bottom wall. It is also pointedout that the resistive heating element 17 can be easily removed andreplaced from the channel of the baffle plate.

With the baffle plate 40 secured in position, as better shown in FIGS. 4and 5, it can be seen that the outer peripheral edge 41 of the baffleplate 40 extends spaced from the inner surface 24 of the tank side wall12 and point in the direction of the cavity zone 23 and spaced above thetop surface 14′ of the dome-shaped bottom wall 14 to create thecircumferential water inlet opening about the baffle plate. As soon asthe lower resistive heating element 17 is provided power, it quicklyheats the water surrounding it in the channel 42 causing the heatedwater to rise upwards through the slot opening 46 drawing water formunder the inverted dome shape baffle plate 40 causing water to be drawnunder the baffle plate through the circumferential passage 57 therebydrawing heated water from the surrounding area of the baffle plate mixedwith hot water released through the slot opening back under the baffleplate and above the cavitated zone 23 to raise the temperature in thatarea well above 46 degrees C. to prevent the proliferation of theLegionella bacteria in this zone which may contain sediments and all thearea in the bottom end 43 of the tank.

With further reference to FIGS. 4 and 5 there is shown a stop bar 51secured under and across the baffle plate circumferential cone wall 52′and spaced immediately under the channel 42. The purpose of the stop bar51 is to prevent the baffle plate 52 from being dislodged by movingupwardly beyond its efficient working position, as the stop bar 51 wouldabut a lower arm 50 of the resistive heating element 17. The stop bar 51is constructed as a tin plastic bar oriented vertically to minimizeobstruction to water flow moving upwards from under the bar. Also, thelower edge 49 of the entrance opening 47 of the channel 42 provides anabutment on the resistive heating element 17 should there be an upwarddisplacement of the baffle plate 52.

With respect to both embodiments, the sensor 33 and the control 32assures the maintenance of the high water temperature in the bottom end30 of the tank. If there is a draw of water from the tank causing thewater temperature in the tank lower end to drop by the introduction ofcold replacement water through the dip tube 25, the lower resistiveheating element 17 will be powered for the water in its region to attainits set point temperature of 60 degrees C. and cause heated water to bedrawn under the baffle plate. However, if the water in the tank is idlefor a long period of time and the water temperature in the bottom endfalls below a value of say 50 degrees C., as sensed by the controller32, the bottom element will be powered by the controller 32 receivingsensed temperature signals from its connection 33′ to the temperaturesensor 33, to raise the temperature in the bottom end 43.

As shown in FIG. 6, it is also conceivable that the outer peripheraledge 41 of the baffle plate be positioned in close friction fit with thetank inner surface 24 and water admitted thereunder through a series ofcut-outs 61 formed spaced-apart along the outer circular peripheral edge41 of the inverted cone shape baffle plate 40 to provide water passagein the area of the cavitated zone 23 about the dome-shaped bottom wall14 when the resistive heating element 17 is powered.

It can be summarized that the above described examples of the preferredembodiment of the present invention provide a novel method of preventingbacteria growth in the bottom region of an electric water heater andparticularly, but not exclusively, one having a dome-shaped bottom wall14 where a cavitated zone 23 is formed about the dome-shaped bottomwall. Briefly described, the method comprises the steps of sensing watertemperature by means of a sensor 33 in the immediate area of thedome-shaped bottom wall to assure that the temperature of water thereinis maintained at a temperature of at least 46 degrees C. The method alsoprovides convection means to convect heated water from above thedome-shaped bottom wall 14 into the area of the cavitated zone 23 andover the dome-shaped bottom wall 14 to maintain the water temperatureabove 46 degrees C. to prevent harmful bacteria growth. The method alsocomprises controlling the duration of the convection of hot water above46 degrees C. in the bottom end of the tank.

The step of convecting heated water to the bottom end 30 of the tank 11can be achieved by the use of a water pump 27 mounted at any convenientlocation on the tank to draw hot water from the upper region 16 of thetank where water is at a higher temperature through conduit means andreleasing the hot water above the dome-shaped bottom wall 14 and intothe area or region of the cavitated zone 23 to raise the watertemperature therein above 46 degrees C. and at a water pump pressuresufficiently low to prevent precipitation of sediment deposits into theupper region of the tank where water is drawn.

The step of convecting may also be achieved, as described herein, bymounting an inverted shallow cone shape baffle plate under the lowerresistive heating element 17 and spaced above the dome-shaped bottomwall 14 a predetermined distance therefrom to direct heated watergenerated by the lower resistive heating element to be partiallyconvected back into the area of the cavitated zone 24 and over thedome-shaped bottom wall 14 to raise the temperature in the bottom end 30of the tank above 46 degrees C. A temperature sensor 33 and a suitablecontrol 32 assures the operation of the lower resistive heating elementto maintain such temperature to prevent bacteria growth in the tankbottom end.

It is within the ambit of the present invention to cover any obviousmodifications of the preferred embodiment described herein provided suchmodifications fall within the scope of the appended claims

1. An electric water heater comprising a tank defined by a cylindricalside wall, a top wall and a dome-shaped bottom wall; a cold water inletdisposed for releasing cold water under pressure in a lower portion ofsaid tank, two or more resistive heating elements to heat water in anupper and lower region of said tank, temperature sensing means forsensing water temperature in said upper and lower regions of said tank,control means to actuate at least one of said two or more resistiveheating elements when said water temperature in at least one of saidregions falls below a set point temperature value as sensed by saidsensing means, said dome-shaped bottom wall defining a surroundingcavitated zone thereabout where water temperature is at its lowest andwater is less agitated and more prone to bacteria proliferation, andconvection means to cause heated water from said tank to be released insaid lower region of said tank and in the immediate area of saiddome-shaped bottom wall and said surrounding bacteria proliferation zoneat a temperature sufficient to kill and prevent bacteria proliferation.2. The electric water heater as claimed in claim 1 wherein saidconvection means is constituted by a water pump having an inlet conduitextending into said upper region of said tank where water is at a hightemperature of at least 46 degrees C., said water pump having an outletconduit connected thereto which has a discharge end disposed to releasehot water from said upper region into said lower region at a controlledpressue not to precipitate any sediment deposits into the upper regionof the tank.
 3. The electric water heater as claimed in claim 2 whereinsaid outlet conduit has a curved conduit end section and oriented tocause a gentle swirling motion to said water discharged at said lowerregion.
 4. The electric water heater as claimed in claim 2 wherein saidoutlet conduit is a dip tube extending into the interior of said tank incontact with hot water therein.
 5. The electric water heater as claimedin claim 4 wherein said dip tube is formed of non-oxidation material,and preferably but not exclusively, plastics material capable ofwithstanding temperatures of above 60 degrees C.
 6. The electric waterheater as claimed in claim 2 wherein said water pump is mounted on a topwall of an outer casing of said water heater and wherein said inlet andoutlet conduits are removably secured to said tank by watertightbushings.
 7. The electric water heater as claimed in claim 2 whereinsaid water in said upper region of said tank is at a temperature of atlest 46 degrees C. and commonly 60 degrees C. wherein water dischargedfrom said discharge end of said outlet conduit is released at atemperature above 46 degrees C. in which Legionella bacteria does notsurvive.
 8. The electric water heater as claimed in claim 1 wherein saidconvection means is constituted by an inverted shallow cone shape baffleplate having an outer circular peripheral edge, said baffle plate beingretained in said lower region of said tank and housing a substantialportion of a lower one of said resistive heating elements extending insaid tank lower region, said resistive heating element being disposed inan upper chamber integrally formed along a top end section of saidbaffle plate and in communication with the space under said baffle platespaced from said dome-shaped bottom wall, said upper chamber having anelongated slot opening for releasing hot water from under said baffleplate heated by said resistive heating element and causing heated waterto flow about said outer circular peripheral edge and under the baffleplate and against said dome-shaped bottom wall.
 9. The electric waterheater as claimed in claim 8 wherein said water passage means is formedby said baffle plate outer circular peripheral edge being disposedclosely spaced to an inner surface of said tank cylindrical side walland a predetermined distance above said surrounding cavitated zone aboutsaid dome-shaped bottom wall to define a circumferential water inletopening about said baffle plate to admit water under said dome when aconvection flow is initiated by operating said lower one of saidresistive heating elements to cause heated water to move upwardly fromunder said baffle plate and thereby drawing water from above saidcircumferential water inlet to move under said baffle plate and increasethe temperature of water therein above 46 degrees C. to destroy anybacteria in the region under said baffle plate in the immediate area ofsaid-shaped bottom wall and said cavitated zone.
 10. The electric waterheater as claimed in claim 8 wherein said water passage means is formedby a series of cut-outs formed spaced-apart along said outer circularperipheral edge of said inverted cone shape baffle plate to providewater passage to the immediate area of said cavitated zone about saiddome-shape bottom wall, said outer circular peripheral edge being infrictional contact with an inner surface of said circumferential sidewall of said tank.
 11. The electric water heater as claimed in claim 8wherein said baffle plate has a stop bar connected thereto and extendingtransversely under said upper chamber.
 12. The electric water heater asclaimed in claim 8 wherein there is further provided spacer memberssecured spaced apart about an outer peripheral section of said invertedcone shape baffle plate and projecting beyond said outer circularperipheral edge for frictional engagement with said inner surface ofsaid tank cylindrical side wall.
 13. The electric water heater asclaimed in claim 12 wherein said spacer members are spring clips havingsmooth resilient contact ends to prevent damage to a glass coating onsaid inner surface of said tank cylindrical side wall.
 14. The electricwater heater as claimed in claim 8 wherein said upper chamber is achannel having opposed parallel vertical side walls, said side wallseach having an inwardly inclined top section which define therebetweensaid elongated slot opening spaced above said lower one of saidresistive heating element housed in said upper channel.
 15. A method ofpreventing bacterial growth in a bottom end of a water holding tank ofan electric water heater in the region of a dome-shaped bottom walldefining a cavitated zone thereabout, said method comprising the stepsof: (i) sensing water temperature in the immediate area of saiddome-shaped bottom wall, (ii) convecting heated water from a regionspaced above said dome-shaped bottom wall and into the area of saidcavitated zone to maintain the temperature of water in said region ofsaid dome-shaped bottom wall and said cavitated zone at a temperaturesufficient to prevent harmful bacteria growth, and (iii) controlling theduration of said step of convecting in relation to said sensedtemperature to maintain water temperature preventing bacteria growth.16. The method as claimed in claim 15 wherein said water holding tankhas a cylindrical side wall, a top wall and said dome-shaped bottomwall; a cold water inlet disposed for releasing cold water underpressure in a lower portion of said tank above said dome-shaped bottomwall, two or more resistive heating elements to heat water in an upperand lower region of said tank, temperature sensing means for sensingwater temperature in said upper and lower regions of said tank, controlmeans to actuate at least one of said two or more resistive heatingelements when said water temperature in at leas one of said regionsfalls below a set point temperature value as sensed by said sensingmeans, and wherein said step (ii) comprises convecting heated water fromsaid tank to raise and maintain the temperature of water in the area ofsaid cavitated zone to at least 46 degrees C.
 17. The method as claimedin claim 16 wherein said step (ii) of convecting comprises using a waterpump to pump water from said upper region of said tank where water is ata high temperature of at least 46 degrees C. through conduit means andreleasing said water at said higher temperature in the immediate area ofsaid dome-shaped bottom wall and said cavitated zone to raise the watertemperature to at least 46 degrees C. to kill the Legionella bacteria.18. The method as claimed in claim 17 wherein said pumped water isreleased in a gentle spiral motion to prevent water temperaturestratification in said lower region of said tank.
 19. The method asclaimed in claim 16 wherein said step (ii) of convecting comprisesmounting an inverted shallow cone shape baffle plate under saidresistive heating element in said lower region of said tank to create acircumferential water passage in the immediate area of said cavitatedzone, said baffle plate having an upper open-ended channel formed in atop portion thereof for housing said resistive heating element in saidlower region of said tank, and causing a convection flow between saidcircumferential water passage and said open-ended channel by operatingsaid resistive heating element causing heated water to flow upwardsthrough said open-ended channel and thereby drawing heated water fromabove said cone shape baffle plate into said circumferential waterpassage and into the immediate area of said cavitated zone to maintainwater temperature in said cavitated zone and under said baffle plate toat least 46 degrees C.